The Kensei Armor comes from a Mythic Tale, The Six Blades of Kojiro, and once belonged to the Straw Hat Kojiro. It raises Resolve gains and Ghost weapon damage, and enemies you tag with a Ghost weapon hit softer and take more damage.
Start The Six Blades of Kojiro and beat the five ronin tied to Kojiro, then defeat Kojiro himself in a duel to claim the armor.
Throw kunai or other Ghost weapons to open a fight, then finish the weakened targets with your sword. The name kensei means sword saint. A tier II version with the Ominous Dawn dye was the first armor shown in the 2018 E3 demo.